Solid electrolytic capacitor containing a conductive polymer

ABSTRACT

A method for forming an electrolytic capacitor is disclosed. The method includes forming a conductive polymer coating by polymerizing a monomer in the presence of less than a stoichiometric amount of an oxidative polymerization catalyst. The present inventor has found that the use of less than the stoichiometric amount of the oxidative polymerization catalyst per mole of monomer can slow the polymerization of the monomer, creating oligomers that are shorter in length than if fully polymerized into a polymer. Without wishing to be bound by theory, it is believed that these shorter oligomers provide better penetration into the porous anode. Thus, the resulting conductive polymer layer can be more intimately positioned with respect to the anode. As a result, the formed capacitor can exhibit better performance.

BACKGROUND OF THE INVENTION

Electrolytic capacitors (e.g., tantalum capacitors) are increasinglybeing used in the design of circuits due to their volumetric efficiency,reliability, and process compatibility. For example, one type ofcapacitor that has been developed is a solid electrolytic capacitor thatincludes an anode (e.g., tantalum), a dielectric oxide film (e.g.,tantalum pentoxide, Ta₂O₅) formed on the anode, a solid electrolytelayer, and a cathode. The solid electrolyte layer may be formed from aconductive polymer, such as described in U.S. Pat. No. 5,457,862 toSakata, et al., U.S. Pat. No. 5,473,503 to Sakata, et al., U.S. Pat. No.5,729,428 to Sakata, et al., and U.S. Pat. No. 5,812,367 to Kudoh, etal. The conductive polymer electrolyte of these capacitors has typicallybeen formed through sequential dipping into separate solutionscontaining the ingredients of the polymer layer. For example, themonomer used to form the conductive polymer is often applied in onesolution, while the catalyst and dopant is applied in a separatesolution or solutions. Such sequential application of the solutions,however, is time consuming and not generally cost effective. Attemptshave been made to use a polymerization solution containing both themonomer and the catalyst. However, such a single solution is not alwayspractical due to the difficulty in achieving an acceptable life span forthe solution. That is, when mixed together in solution with theoxidative polymerization catalyst, the monomer tends to prematurelyinitiate polymerization while still in solution and prior to applicationto the anode part. This premature polymerization may lead to anincreased number processing steps and ultimately degrade the conductivepolymer layer.

As such, a need currently exists for an improved method for forming aconductive polymer layer on an electrolytic capacitor from apolymerization solution.

SUMMARY OF THE INVENTION

In accordance with one embodiment of the present invention, a method forforming a solid electrolytic capacitor is disclosed. The methodcomprises forming an anode that contains a valve-action metalcomposition; anodizing a surface of the anode to form a dielectriclayer; and forming a conductive polymer coating over the anodized anodeby polymerizing a monomer in the presence of an oxidative polymerizationcatalyst. Less than a stoichiometric amount of the oxidativepolymerization catalyst is present in the polymerization solution permole of monomer. A dopant may also be present.

Other features and aspects of the present invention are set forth ingreater detail below.

DETAILED DESCRIPTION OF REPRESENTATIVE EMBODIMENTS

It is to be understood by one of ordinary skill in the art that thepresent discussion is a description of exemplary embodiments only, andis not intended as limiting the broader aspects of the presentinvention, which broader aspects are embodied in the exemplaryconstruction.

Generally speaking, the present invention is directed to a method forforming an electrolytic capacitor. The method includes forming aconductive polymer coating by polymerizing a monomer in the presence ofless than a stoichiometric amount of an oxidative polymerizationcatalyst. The present inventor has found that the use of less than thestoichiometric amount of the oxidative polymerization catalyst per moleof monomer can slow the polymerization of the monomer, creatingoligomers that are shorter in length than if fully polymerized into apolymer. Without wishing to be bound by theory, it is believed thatexcess monomer etches oligomers and provides better penetration into theporous anode. Thus, the resulting conductive polymer layer can be moreintimately positioned with respect to the anode. As a result, the formedcapacitor can exhibit better performance.

In one particular embodiment, the conductive polymer is formed from apolymerization solution of both a monomer, an oxidative polymerizationcatalyst, and a dopant. The formation of a conductive polymer layerusing a polymerization solution, compared with applying the conductivemonomer and the oxidative polymerization catalyst/dopant in separatesolutions, can reduce processing steps and can allow for better controlof the polymerization reaction stoichiometry. As the present inventorhas discovered, the conductive polymer layer formed from apolymerization solution can form an electrolytic capacitor havingdecreased ESR, especially at high humidity and/or high temperatures.

The solid electrolytic capacitor of the present invention generallycontains an anode formed from a valve metal composition. The valve metalcomposition may have a high specific charge, such as about 5,000microFarads*Volts per gram (“μF*V/g”) or more, in some embodiments about10,000 μF*V/g or more, in some embodiments from about 15,000 μF*V/g toabout 250,000 μF*V/g or more. The valve metal composition contains avalve metal (i.e., metal that is capable of oxidation) or valvemetal-based compound, such as tantalum, niobium, aluminum, hafnium,titanium, alloys thereof, oxides thereof, nitrides thereof, and soforth. For example, the anode may be formed from a valve metal oxidehaving an atomic ratio of metal to oxygen of 1: less than 25, in someembodiments 1: less than 2.0, in some embodiments 1: less than 1.5, andin some embodiments, 1:1. Examples of such valve metal oxides mayinclude niobium oxide (e.g., NbO), tantalum oxide, etc., and aredescribed in more detail in U.S. Pat. No. 6,322,912 to Fife, which isincorporated herein in its entirety by reference thereto for allpurposes.

Conventional fabricating procedures may generally be utilized to formthe anode. In one embodiment, a tantalum or niobium oxide powder havinga certain particle size is first selected. The particle size may varydepending on the desired voltage of the resulting electrolytic capacitorelement. For example, powders with a relatively large particle size(e.g., about 10 micrometers) are often used to produce high voltagecapacitors, while powders with a relatively small particle size (e.g.,about 0.5 micrometers) are often used to produce low voltage capacitors.The particles are then optionally mixed with a binder and/or lubricantto ensure that the particles adequately adhere to each other whenpressed to form the anode. Suitable binders may include camphor, stearicand other soapy fatty acids, Carbowax (Union Carbide), Glyptal (GeneralElectric), polyvinyl alcohols, napthaline, vegetable wax, and microwaxes(purified paraffins). The binder may be dissolved and dispersed in asolvent. Particularly suitable solvents include water and alcohols. Whenutilized, the percentage of binders and/or lubricants may vary fromabout 0.1% to about 8% by weight of the total mass. It should beunderstood, however, that binders and lubricants are not required in thepresent invention. Once formed, the powder is compacted using anyconventional powder press mold. For example, the press mold may be asingle station compaction press using a die and one or multiple punches.Alternatively, anvil-type compaction press molds may be used that useonly a die and single lower punch. Single station compaction press moldsare available in several basic types, such as cam, toggle/knuckle andeccentric/crank presses with varying capabilities, such as singleaction, double action, floating die, movable platen, opposed ram, screw,impact, hot pressing, coining or sizing. The powder may be compactedaround an anode wire (e.g., tantalum wire). It should be furtherappreciated that the anode wire may alternatively be attached (e.g.,welded) to the anode subsequent to pressing and/or sintering of theanode.

After compression, any binder/lubricant may be removed by heating thepellet under vacuum at a certain temperature (e.g., from about 150° C.to about 500° C.) for several minutes. Alternatively, thebinder/lubricant may also be removed by contacting the pellet with anaqueous solution, such as described in U.S. Pat. No. 6,197,252 toBishop, et al., which is incorporated herein in its entirety byreference thereto for all purposes. Thereafter, the pellet is sinteredto form a porous, integral mass. For example, in one embodiment, thepellet may be sintered at a temperature of from about 1200° C. to about2000° C., and in some embodiments, from about 1500° C. to about 1800° C.under vacuum. Upon sintering, the pellet shrinks due to the growth ofbonds between the particles. In addition to the techniques describedabove, any other technique for forming the anode may also be utilized inaccordance with the present invention, such as described in U.S. Pat.No. 4,085,435 to Galvagni; U.S. Pat. No. 4,945,452 to Sturmer, et al.;U.S. Pat. No. 5,198,968 to Galvagni; U.S. Pat. No. 5,357,399 toSalisbury; U.S. Pat. No. 5,394,295 to Galvagni, et al.; U.S. Pat. No.5,495,386 to Kulkarni; and U.S. Pat. No. 6,322,912 to Fife, which areincorporated herein in their entirety by reference thereto for allpurposes.

Regardless of the particular manner in which it is form, the thicknessof the anode may be selected to improve the electrical performance ofthe electrolytic capacitor element. For example, the thickness of theanode (in the -z direction in FIG. 1) may be about 4 millimeters orless, in some embodiments, from about 0.2 to about 3 millimeters, and insome embodiments, from about 0.4 to about 2 millimeters. Such arelatively small anode thickness (i.e., “low profile”) helps dissipateheat generated by the high specific charge powder and also provide ashorter transmission path to minimize ESR and inductance. The shape ofthe anode may also be selected to improve the electrical properties ofthe resulting capacitor. For example, the anode may have a shape that iscurved, sinusoidal, rectangular, U-shaped, V-shaped, etc. The anode mayalso have a “fluted” shape in that it contains one or more furrows,grooves, depressions, or indentations to increase the surface to volumeratio to minimize ESR and extend the frequency response of thecapacitance. Such “fluted” anodes are described, for instance, in U.S.Pat. No. 6,191,936 to Webber, et al.; U.S. Pat. No. 5,949,639 to Maeda,et al.; and U.S. Pat. No. 3,345,545 to Bourgault et al., as well as U.S.Patent Application Publication No. 2005/0270725 to Hahn, et al., all ofwhich are incorporated herein in their entirety by reference thereto forall purposes.

The anode may be anodized so that a dielectric layer is formed over andwithin the porous anode. Anodization is an electrical chemical processby which the anode metal is oxidized to form a material having arelatively high dielectric constant. For example, a tantalum anode maybe anodized to form tantalum pentoxide (Ta₂O₅), which has a dielectricconstant “k” of about 27. The anode may be dipped into a weak acidsolution (e.g., phosphoric acid) at an elevated temperature (e.g., about60° C.) that is supplied with a controlled amount of voltage and currentto form a tantalum pentoxide coating having a certain thickness. Thepower supply is initially kept at a constant current until the requiredformation voltage is reached. Thereafter, the power supply is kept at aconstant voltage to ensure that the desired dielectric quality is formedover the surface of the tantalum pellet. The anodization voltagetypically ranges from about 5 to about 200 volts, and in someembodiments, from about 20 to about 100 volts. In addition to beingformed on the surface of the anode, a portion of the dielectric oxidefilm will also typically form on the surfaces of the pores. It should beunderstood that the dielectric layer may be formed from other types ofmaterials and using different techniques.

A protective adhesive layer may optionally be formed over the dielectriclayer to help adhere the dielectric layer to the cathode layers. Theprotective adhesive layer can generally include a variety of materialsthat are capable of forming a thin coating and that can improve theelectrical performance of the resulting capacitor. In one particularembodiment, the protective adhesive layer may include, for instance, apolymer containing a repeating unit having a functional hydroxyl group.As such, the resulting polymer can have at least two hydroxyl groups inthe polymer chain. Examples of polymers having at least two hydroxylgroups may include polyvinyl alcohol (“PVA”), copolymers of polyvinylalcohol (e.g., ethylene vinyl alcohol copolymers, methyl methacrylatevinyl alcohol copolymers, etc.), polysaccharides, etc.

Vinyl alcohol polymers, for instance, have at least two or more vinylalcohol units in the molecule and may be a homopolymer of vinyl alcohol,or a copolymer containing other monomer units. Vinyl alcoholhomopolymers may be obtained by hydrolysis of a vinyl ester polymer,such as vinyl formate, vinyl acetate, vinyl propionate, etc. Vinylalcohol copolymers may be obtained by hydrolysis of a copolymer of avinyl ester with an olefin having 2 to 30 carbon atoms, such asethylene, propylene, 1-butene, etc.; an unsaturated carboxylic acidhaving 3 to 30 carbon atoms, such as acrylic acid, methacrylic acid,crotonic acid, maleic acid, fumaric acid, etc., or an ester, salt,anhydride or amide thereof; an unsaturated nitrile having 3 to 30 carbonatoms, such as acrylonitrile, methacrylonitrile, etc.; a vinyl etherhaving 3 to 30 carbon atoms, such as methyl vinyl ether, ethyl vinylether, etc.; and so forth.

The use of vinyl alcohol copolymers may be particularly desired in thepresent invention to optimize adhesion properties of the protectiveadhesive layer to the dielectric layer and solid electrolyte. An acrylicor methacrylic ester, for instance, may be copolymerized with a vinylester to provide a hydrophilic polymer having excellent adhesionproperties. Suitable esters of acrylic acid or methacrylic acid mayinclude esters of unbranched or branched alcohols having from 1 to 15carbon atoms. Preferred methacrylic esters or acrylic esters are methylacrylate, methyl methacrylate, ethyl acrylate, ethyl methacrylate,propyl acrylate, propyl methacrylate, n-, iso- and t-butyl acrylate, n-,iso- and t-butyl methacrylate, 2-ethylhexyl acrylate, etc. The fractionof these comonomers may range from about 30 mol % to about 70 mole %,and in some embodiments, from about 40 mol % to about 60 mol % of thevinyl alcohol copolymer.

Regardless of the monomers employed, the degree of hydrolysis may beselected to optimize the protective adhesive layer properties of thepolymer. For example, the degree of hydrolysis may be about 90 mole % orgreater, in some embodiments about 95 mole % or greater, and in someembodiments, about 98 mole % or more. For a vinyl alcohol homopolymer,this would mean that about 90 mole % or greater, in some embodimentsabout 95 mole % or greater, and in some embodiments, about 98 mole % ormore of the acetate groups on the parent polymer are hydrolyzed. Such anelevated degree of hydrolysis lowers the solubility of the polymer inwater, while preserving its hydrophilic properties. Thus, it is believedthat after formation of the protective adhesive layer that includes ahighly hydrolyzed polymer, the protective adhesive layer can absorb agreater amount of water while remaining a solid coating. Thus, theprotective adhesive layer can help prevent water molecules fromcontacting the anode and the dielectric layer by absorbing watermolecules from the air, especially in an environment having a highrelative humidity. Examples of suitable highly hydrolyzed polyvinylalcohol polymers are available under the trade name Mowiol® from KuraraySpecialties Europe GmbH, Frankfurt, such as Mowiol® 3-98, Mowiol® 4-98,and Mowiol® 6-98.

For example, the presence of hydroxyl groups in the hydrophilic polymermay provide adhesive characteristics to the protective adhesive layerthat help bond to the dielectric layer to the conductive polymer. Forexample, without wishing to be bound by theory, it is believed that thehydroxyl groups can increase the adhesion of the layers throughattraction and/or bonds (e.g., van Der Waals forces, hydrogen bonding,ionic bonds, etc.).

Other materials may also be employed to improve the adhesive nature ofthe barrier. Examples of such materials include acrylate or methacrylatepolymers, such as polymethyl methacrylate, polyethyl methacrylate,polypropyl methacrylate, polybutyl methacrylate,hydroxyethylmethacrylate, etc.; polyurethane; polystyrene; esters ofunsaturated or saturated fatty acids (e.g., glycerides); etc. Forinstance, suitable esters of fatty acids include, but are not limitedto, esters of lauric acid, myristic acid, palmitic acid, stearic acid,eleostearic acid, oleic acid, linoleic acid, linolenic acid, aleuriticacid, shellolic acid, and so forth. These esters of fatty acids havebeen found particularly useful when used in relatively complexcombinations to form a “drying oil”, which allows the resulting film torapidly polymerize into a stable layer. Such drying oils may includemono-, di-, and/or tri-glycerides, which have a glycerol backbone withone, two, and three, respectively, fatty acyl residues that areesterified. For instance, some suitable drying oils that may be usedinclude, but are not limited to, olive oil, linseed oil, castor oil,tung oil, soybean oil, and shellac. These and other resinous materialsare described in more detail in U.S. Pat. No. 6,674,635 of Fife, et al.and U.S. Pat. No. 6,864,147 of Fife, et al., both of which areincorporated herein in their entirety by reference thereto for allpurposes.

The material(s) of the protective adhesive layer are typically moreresistive than the conductive polymer that of the solid electrolyte. Forexample, the protective adhesive layer may contain a material having aresistivity of greater than about 0.05 ohm-cm, in some embodimentsgreater than about 5, in some embodiments greater than about 1,000ohm-cm, in some embodiments greater than about 1×10⁵ ohm-cm, and in someembodiments, greater than about 1×10¹⁰ ohm-cm. Despite possessing suchinsulative properties the protective adhesive layer does not typicallyhave a significant adverse effect on the electrical performance of thecapacitor. One reason for this is due to the relatively small thicknessof the barrier, which is normally about 100 micrometers or less, in someembodiments about 50 micrometers or less, and in some embodiments, about10 micrometers or less.

The protective adhesive layer can be applied in a variety of differentways. For example, in one embodiment, the anode part or slug can bedipped into a dipping solution of the desired protective adhesive layermaterial(s). The solution may be formed by dissolving or dispersing thematerials in a solvent. The solvent is also useful in controlling theviscosity of the solution, thereby facilitating the formation of thinlayers. Any solvent of a variety of solvents may be employed, such aswater; glycols (e.g., propylene glycol, butylene glycol, triethyleneglycol, hexylene glycol, polyethylene glycols, ethoxydiglycol, anddipropyleneglycol); glycol ethers (e.g., methyl glycol ether, ethylglycol ether, and isopropyl glycol ether); ethers (e.g., diethyl etherand tetrahydrofuran); alcohols (e.g., methanol, ethanol, n-propanol,iso-propanol, and butanol); triglycerides; ketones; esters (e.g., ethylacetate, butyl acetate, diethylene glycol ether acetate, andmethoxypropyl acetate); amides (e.g., dimethylformamide,dimethylacetamide, dimethylcaprylic/capric fatty acid amide andN-alkylpyrrolidones); nitriles (e.g., acetonitrile, propionitrile,butyronitrile and benzonitrile); sulfoxides or sulfones (e.g., dimethylsulfoxide (DMSO) and sulfolane); and so forth. One particular benefit ofthe present invention is that aqueous solvents (e.g., water) may beemployed due to the hydrophilic nature of the protective adhesive layerpolymers. In fact, water may constitute about 20 wt. % or more, in someembodiments, about 50 wt. % or more, and in some embodiments, about 75wt. % to 100 wt. % of the solvent(s) used in the solution.

Once formed, the anode part can be dipped into the dipping solution oneor more times, depending on the desired thickness of the protectiveadhesive layer. The number of layers that form the protective adhesivelayer can be from about 2 to about 10 layers, and in some embodiments,from about 3 to about 7 layers. Besides dipping, it should also beunderstood that other conventional application methods, such assputtering, screen printing, electrophoretic coating, electron beamdeposition, vacuum deposition, spraying, and so forth, can also be usedto deposit the protective adhesive layer. After forming the protectiveadhesive layer, it is often desired that the anode part be dried tofacilitate evaporation of any solvent used during application.Typically, each layer is dried at a temperature ranging from about 30°C. to about 300° C., and in some embodiments, from about 50° C. to about150° C., for a time period ranging from about 1 minute to about 60minutes, and in some embodiments, from about 15 minutes to about 30minutes. It should also be understood that heating need not be utilizedafter application of each layer, but may instead be utilized only afterformation of the entire protective adhesive layer.

A solid electrolyte is then formed over the anode. According to thepresent invention, the solid electrolyte includes one or more conductivepolymers to form a conductive polymer layer. However, less than thenormally required stoichiometric amount of the oxidative polymerizationcatalyst can be used per mole of monomer in forming the conductivepolymer layer. For example, from about two-fourths to aboutthree-fourths of the normally required stoichiometric amount of theoxidative polymerization catalyst can be used per mole of monomer informing the conductive polymer layer, such as about half of thestoichiometric amount.

The present inventor has found that the use of less than thestoichiometric amount of the oxidative polymerization catalyst per moleof monomer can slow the polymerization of the monomer, creatingoligomers that are shorter than if fully polymerized into a polymer.Without wishing to be bound by theory, it is believed that the excessmonomer etches oligomers which provides better penetration into theporous anode. Thus, the resulting conductive polymer layer can be moreintimately positioned with respect to the anode. As a result, the formedcapacitor can exhibit better performance.

For instance, when the monomer includes 3,4-ethylenedioxy thiophene, thenormally required molar ratio used to polymerize 3,4-ethylenedioxythiophene into PEDT is about 1 mole 3,4-ethylenedioxy thiophene to 18moles oxidative polymerization catalyst. However, less than 18 moles ofoxidative polymerization catalyst can be present in the polymerizationsolution per mole of monomer (e.g., 3,4-ethylenedioxy thiophene), suchas less than about 15 moles of oxidative polymerization catalyst permole of monomer. For example, from about 5 to about 15 moles, or fromabout 5 to about 12 moles, of oxidative polymerization catalyst can bepresent in the polymerization solution per mole of monomer, such asabout 10 moles of oxidative polymerization catalyst per mole of monomer.

In a preferred embodiment, the conductive polymer layer is formed on theanode from a polymerization solution of both a monomer and an oxidativepolymerization catalyst. The formation of a conductive polymer layerusing a polymerization solution, compared with applying the conductivemonomer and the oxidative polymerization catalyst in separate solutions,can reduce processing steps and can allow for better control of thepolymerization reaction stoichiometry. The conductive polymer layer canbe formed on the dielectric layer or the optional protective adhesivelayer.

Suitable conductive polymers include, but are not limited to,polypyrroles; polythiophenes, such as poly(3,4-ethylenedioxy thiophene)(PEDT); polyanilines; polyacetylenes; poly-p-phenylenes; and derivativesthereof. If desired, the solid electrolyte can be formed from multipleconductive polymer layers, such as one layer formed from PEDT andanother layer formed from a polypyrrole. Any suitable monomer(s) may beemployed to form the conductive polymer. For example, 3,4-ethylenedioxythiophene (BAYTRON M, Bayer Corp.) may be used as a monomer forforming PEDT. An oxidative polymerization catalyst may be employed toinitiate the polymerization of the monomer(s). The oxidativepolymerization catalyst can be any transitional metal salt useful as anoxidizing agent, such as those transitional metal salts derivatized withorganic ligands. A preferred oxidative polymerization catalyst can be anorganic acid ligand combined with iron (III), such as iron (III)tosylate. One suitable oxidative polymerization catalyst is BAYTRON C,which is iron (III) toluene-sulphonate and n-butanol and sold by BayerCorporation.

However, when mixed together in solution, a small portion of themonomer(s) tends to polymerize, even absent the application of heat. Thepresent inventor has discovered, however, that such prematurepolymerization may be substantially inhibited through the appropriateselection of a polar solvent that functions as a reaction inhibitor. Inone particular embodiment, an aprotic polar solvent capable of donatingelectrons can be included in the polymerization solution. Withoutwishing to be bound by theory, it is believed that the localizednegative charge on a polar solvent can attract, through electrondonation (e.g., acid-base reactions), the positively charged metal(e.g., iron III) of the oxidative polymerization catalyst to form aweakly bonded complex. This weak complex may effectively inhibit theability of the oxidative polymerization catalyst to oxidize the monomerfor polymerization. As such, only a relatively small amount, if any, ofthe monomer is prematurely polymerized in the polymerization solutionprior to its application to the electrolyte capacitor. Additionally, thelife span on the polymerization solution can be greatly extended.

Additionally, polar solvents, such as aprotic solvents, can act as adissolve any oligomers prematurely formed while still in thepolymerization solution. Thus, the oligomers can be inhibited fromfurther polymerization, and the shelf life of the polymerizationsolution can be extended, even if oligomers are prematurely formed. Assuch, the combination of the polar solvent and the monomer with lessthan a stoichiometric amount of the oxidative polymerization catalystcan provide further advantages to the method of producing the conductivepolymer layer.

Particularly suitable polar solvents are aprotic solvents, such asdipolar aprotic solvents, which lack an acidic proton. Polar aproticsolvents include, but are not limited to, N-methylpyrrolidone, dimethylsulfoxide, dimethylformamide, hexamethylphosphorotriamide, dimethylacetamide, methyl ethyl ketone, and so forth.

In most embodiments, the polar solvent(s) are combined with one or moreco-solvents to form a solvent system for the solution. In suchembodiments, the weight ratio of the co-solvents(s) to the polarsolvent(s) may be about 50:1 or more, in some embodiments from about50:1 to about 250:1, and in some embodiments, from about 75:1 to about150:1. For example, the polar solvent(s) may constitute from about 0.001wt. % to about 10 wt. %, in some embodiments, from about 0.01 wt. % toabout 5 wt. %, and in some embodiments, from about 0.05 wt. % to about 1wt. % of the polymerization solution. Likewise, the co-solvent(s) mayconstitute from about 20 wt. % to about 90 wt. %, in some embodiments,from about 30 wt. % to about 80 wt. %, and in some embodiments, fromabout 40 wt. % to about 60 wt. % of the polymerization solution. It isbelieved that such a small amount of polar solvent in the completesolvent system of the polymerization solution can sufficiently inhibitpremature polymerization, while still allowing polymerization onceapplied to the anode.

Any suitable co-solvent that is miscible with the polar solvent may beemployed in the present invention. Exemplary co-solvents may includeglycols (e.g., propylene glycol, butylene glycol, triethylene glycol,hexylene glycol, polyethylene glycols, ethoxydiglycol, anddipropyleneglycol); glycol ethers (e.g., methyl glycol ether, ethylglycol ether, and isopropyl glycol ether); ethers (e.g., diethyl etherand tetrahydrofuran); alcohols (e.g., methanol, ethanol, n-propanol,iso-propanol, and butanol); triglycerides; ketones; esters (e.g., ethylacetate, butyl acetate, diethylene glycol ether acetate, andmethoxypropyl acetate); amides (e.g., dimethylformamide,dimethylacetamide, dimethylcaprylic/capric fatty acid amide andN-alkylpyrrolidones); nitriles (e.g., acetonitrile, propionitrile,butyronitrile and benzonitrile); sulfoxides or sulfones (e.g., dimethylsulfoxide (DMSO) and sulfolane); and so forth. Particularly suitableco-solvent(s) are aliphatic alcohols, such as ethanol, propanol,methanol, isopropanol, butanol, and so forth.

The polymerization solution may also contain a dopant. The dopant may bean oxidizing or reducing agent, and can provide excess charge to theconductive polymer. For example, in one embodiment, the dopant can beany conventional anion. Particularly, the ions of aromatic sulfonicacids, aromatic polysulfonic acids, organic sulfonic acids including ahydroxy group, organic sulfonic acids including a carboxyl group,alycyclic sulfonic acids, benzoquinone sulfonic acids and other organicsulfonic acids can effectively stabilize the conductivity of aconducting polymer layer because their molecule sizes are large enoughto obstruct easy dedoping in a high temperature atmosphere. Examples ofsuch organic sulfonic acids are dodecylbenzene sulfonic acid, toluenesulfonic acid, benzyl sulfonic acid, naphthalene sulfonic acid, phenolsulfonic acid, sulfoisofuthalic acid, sulfosalicylic acid, camphorsulfonic acid, and adamantane sulfonic acid. In one embodiment, thedopant can be supplied from the same compound as the oxidativepolymerization catalyst. For instance, iron III toluene-sulphonate cansupply both the dopant (anion of toluene-sulphonate) and the oxidativepolymerization catalyst (cation of iron III).

A binder may also be employed in the polymerization solution tofacilitate adherence of the solid electrolyte to the dielectric layer.For example, the polymerization solution may contain organic binderswhich are soluble in organic solvents, such as poly(vinyl acetate),polycarbonate, poly(vinyl butyrate), polyacrylates, polymethacrylates,polystyrene, polyacrylonitrile, poly(vinyl chloride), polybutadiene,polyisoprene, polyethers, polyesters, silicones, and pyrrole/acrylate,vinyl acetate/acrylate and ethylene/vinyl acetate copolymers each ofwhich are soluble in organic solvents. It is also possible to usewater-soluble binders such as polyvinyl alcohols as thickeners.Alternatively, those resinous material disclosed above in relation tothe protective adhesive layers can be included in the polymerizationsolution as an organic binder.

Once the polymerization solution is formed, it may be applied to theanode part using any known technique. For instance, conventionaltechniques such as sputtering, screen-printing, dipping, electrophoreticcoating, electron beam deposition, spraying, and vacuum deposition, canbe used to form the conductive polymer coating. Although various methodshave been described above, it should be understood that any other methodfor applying the conductive coating(s) to the anode part can also beutilized in the present invention. For example, other methods forapplying such conductive polymer coating(s) may be described in U.S.Pat. No. 5,457,862 to Sakata, et al., U.S. Pat. No. 5,473,503 to Sakata,et al., U.S. Pat. No. 5,729,428 to Sakata, et al., and U.S. Pat. No.5,812,367 to Kudoh, et al., which are incorporated herein in theirentirety by reference thereto for all purposes. Regardless of theapplication technique employed, the polymerization solution may becooled to further stabilize the polymerization solution and preventpremature polymerization of the monomer(s). For example, thepolymerization solution can be applied at a temperature of less thanabout 20° C., in some embodiments less than about 15° C., in someembodiments less than about 10° C., and in some embodiments, less thanabout 5° C.

Once applied, the conductive polymer may be healed. Healing may occurafter each application of a conductive polymer layer or may occur afterthe application of the entire conductive polymer coating. In someembodiments, the conductive polymer can be healed by dipping the sluginto an electrolyte solution, and thereafter applying a constant voltageto the solution until the current is reduced to a preselected level. Ifdesired, such healing can be accomplished in multiple steps. Forexample, an electrolyte solution can be a dilute solution of themonomer, the catalyst, and dopant in an alcohol solvent (e.g., ethanol).

After application of some or all of the layers described above, the slugmay then be washed if desired to remove various byproducts, excesscatalysts, and so forth. Further, in some instances, drying may beutilized after some or all of the dipping operations described above.For example, drying may be desired after applying the catalyst and/orafter washing the slug in order to open the pores of the slug so that itcan receive a liquid during subsequent dipping steps.

Once the solid electrolyte is formed, the part may then be applied witha carbon coating (e.g., graphite) and silver coating, respectively. Thesilver coating may, for instance, act as a solderable conductor, contactlayer, and/or charge collector for the capacitor and the carbon coatingmay limit contact of the silver coating with the solid electrolyte. Leadelectrodes may then be provided as is well known in the art. Typically,the silver coating includes silver and an organic binder, such as aresin (e.g., an epoxy resin).

Optionally, the formed capacitor can be coated with a barrier layer tohelp protect the capacitor from changes in its working environment. Forinstance, the barrier layer can enable the electrolytic capacitor toincrease it performance in relatively high humidity and/or hightemperature environments. The barrier layer can be positioned betweenthe graphite coating and the silver coating. Alternatively, the barrierlayer can be preferably positioned on the external surface of the silvercoating to form the outermost layer of the resulting capacitor.

The barrier layer can include a barrier polymer configured to reduceoxygen and moisture permeability of the electrolyte capacitor withoutsubstantially affecting the functionality of the coated capacitor. Forexample, the barrier polymer can block passage of oxygen and/or moisturethrough the barrier layer by forming a structure that presents atortuous path for water and/or oxygen molecules to travel. Thus, thebarrier polymer can slow the transmission of the water and/or oxygenmolecules through the barrier layer.

Additionally, the barrier polymer can adhere the barrier layer to thecapacitor, such as to the silver coating having an organic binder. Assuch, the barrier layer can be intimately applied to the capacitorwithout substantially affecting the capacitor's performance. Forexample, the barrier polymer can bond (e.g., ionic bonding, hydrogenbonding, van Der Walls attraction, etc.) to the organic binder of thesilver coating to secure the barrier layer to the capacitor. In someembodiments, these chemical bonds can be initiated from functionalhydroxyl groups positioned on the barrier polymer. The functionalhydroxyl groups can, for instance, be provided from at least twohydroxyl groups (e.g., a polyol) or at least two alkoxy groups.

In one particular embodiment, the barrier polymer can be a polyurethanehaving multiple hydroxyl groups (e.g., a polyurethane diol). Forexample, the polyurethane can be selected from the family of polyetherpolyols, which have multiple hydroxyl groups capable of bonding to theorganic binder of the silver coating. Additionally, polyurethanepolymers generally have good barrier properties, and are generallystable in high temperature and/or high humidity environments. Analternative group of barrier polymers can be polyesters havingfunctional alkoxy groups capable of bonding to the organic binder of thesilver coating, similar to the functional hydroxyl groups discussedabove.

In some embodiments, the barrier and adhesive properties of the barrierpolymer can be increased by adding a polyfunctional crosslinking agentto produce the barrier layer. The addition of a polyfunctionalcrosslinking agent can provide improvements in adhesion, heatresistance, water and moisture resistance, and oxygen resistance to thebarrier layer.

For example, the polyfunctional crosslinking agent can be anitrogen-containing polymer. For example, a polymer containingpolyfunctional aziridine groups can be utilized as a polyfunctionalcrosslinking agent. The term “aziridine” as used herein refers to analkyleneimine group, and “polyfunctional aziridine” includes compoundsproduced by the polymerization of an alkyleneimine, such asethyleneimine, ethylethyleneimine, propyleneimine, and mixtures andderivatives thereof. As such, the polyfunctional aziridine can includepolyalkyleneimine polymers (e.g., polyethyleneimine,polyethylethyleneimine, and polypropyleneimine) or copolymers, and theirderivatives. In one particular embodiment, the polyfunctionalcrosslinking agent can include polyethylenenimine, such as branchedpolyethyleneimine.

Without wishing to be bound by theory, it is believed thatpolyfunctional crosslinking agent can promote adhesion of the barrierlayer to the capacitor by crosslinking the barrier polymer of thebarrier layer with itself and to the organic binder of the silvercoating. Also, the crosslinked barrier polymer can have increasedbarrier properties due to its crosslinked chemical structure thatprovides a more tortuous path through the barrier layer for the water oroxygen molecules. Also, crosslinking the barrier polymer provides thebarrier layer with increased mechanical strength due to the crosslinkedchemical structure of the barrier polymer and/or the polyfunctionalcrosslinking agent.

In one embodiment, polyalkyleneimine polymers (e.g., polyethyleneimine)are preferred due to their ability to form crosslinked structuresthemselves, in addition to crosslinking the polyurethane. For example,branched polyethyleneimine generally containes primary, secondary, andtertiary amines. These amine groups can provide bonding sites formingintermolecular bonds (e.g., hydrogen bonds, van Der Waals bonds, and/orionic bonds) with other amine groups, with functional groups of thepolyurethane, and possibly with any functional groups located on thesurface of the capacitor. For example, when the barrier layer is appliedto a silver or graphite coating containing an organic binder, thepolyfunctional crossliking agent can chemically attract, or possiblyeven bond, polymers of the barrier layer to the organic binder of thesilver and/or graphite coatings. Additionally, polyalkyleneiminepolymers are relatively polar polymers, allowing them to reduce thesurface tension of any applied liquid (e.g., water vapor) to the barrierlayer.

Another nitrogen-containing polymer useful as a polyfunctionalcrosslinking agent includes polyamides and derivatives and copolymersthereof. For instance, one particular polyamide useful as apolyfunctional crosslinking agent can be a polyamideimide. Otherpolyfunctional crosslinking agents can include polyfunctional isocyanatecompounds having at least two or more isocyanate groups. Representativeorganic diisocyanates suitable for the primer coating are aromaticdiisocyanates such as 2,4 tolylene diisocyanate,methylene-bis-p,p′-phenylene diisocyanate, 1,5-naphthalene diisocyanate,polymethylene diisocyanate such as tetramethylene diisocyanate,pentamethylene diisocyanate, hexamethylene diisocyanate, decamethylenediisocyanate, cycloalkylene diisocyanate such as cyclohexylene1,4-diisocyanate, diisocyanates containing heteroatoms in the chain, andmixed isocyanates-isothiocyanates such as 1-isocyanate,6-isothiocyanatehexane. Other examples include toluenediisocyanate (TDI),triphenylmethanetriisocyanate (TTT), isophoronediisocyanate (IPDI),tetramethylxylenediisocyanate (TMXDI) or polymers or derivativesthereof.

If desired, the barrier layer may include other auxiliary substanceswhich may be added to the final composition in relative amounts in orderto impart desirable properties or to suppress undesirable properties.Examples of such substances include viscosity modifiers, dispersant,fillers, plasticizers, pigments, dyes, wetting agents, heat stabilizers,carbon black, silica sols, leveling agents, antifoaming agents,UV-stabilizers and the like. The composition may also be blended withother polymer dispersions such as polyvinyl acetate, epoxy resins,polyethylene, polybutadiene, polyvinyl chloride, polyacrylate and otherhomopolymer and copolymer dispersions.

Of course, the barrier layer is not limited to those materials describedabove. For example, the barrier layer may include any suitable materialuseful for inhibiting the passage of oxygen and/or water through thelayer. For instance, any resinous material (e.g., epoxy) can be utilizedto form a barrier layer.

Since permeability is a function of diffusion, a thicker coat weightslows permeability. Thus, varying the film thickness affects the oxygenand moisture vapor transmission rate. However, the advantage of athicker barrier layer must be weighed against any adverse affect that athicker coat may impart on the performance of the resulting coatedcapacitor.

Thus, as a result of the present invention, a capacitor may be formedthat exhibits excellent electrical properties. For example, a capacitorof the present invention typically has an ESR less than about 1000milliohms (mohms), in some embodiments less than about 500 mohms, and insome embodiments, less than about 100 mohms. The equivalent seriesresistance of a capacitor generally refers to the extent that thecapacitor acts like a resistor when charging and discharging in anelectronic circuit and is usually expressed as a resistance in serieswith the capacitor.

Additionally, when a capacitor is formed having a barrier layer asdescribed above, the resulting capacitor can have an ESR less than about1000 milliohms (mohms), in some embodiments less than about 500 mohms,and in some embodiments, less than about 125 mohms, even after aging for1000 hours at 85° C. and at 85% relative humidity. Thus, a capacitorformed having a barrier layer as described above can have a relativelysmall change in ESR after aging for 1000 hours at 85° C. and at 85%relative humidity (when compared to its ESR before aging), such as lessthan 500%, in some embodiments, less than 100%, and in some embodimentsless than 25%.

In addition, after the anode is healed through the application ofvoltage, the resulting leakage current, which generally refers to thecurrent flowing from one conductor to an adjacent conductor through aninsulator, can be maintained at relatively low levels due to themechanical stability of the interface provided by the protectiveadhesive layer. For example, the numerical value of the normalizedleakage current of a capacitor of the present invention is, in someembodiments, less than about 0.1 μA/μF*V, in some embodiments less thanabout 0.01 μA/μF*V, and in some embodiments, less than about 0.001μA/μF*V, where μA is microamps and uF*V is the product of thecapacitance and the rated voltage.

The present invention may be better understood by reference to thefollowing examples.

Test Procedures

Equivalent Series Resistance (ESR), Capacitance, and Dissipation Factor:

Equivalence series resistance and impedance were measured using aKeithley 3330 Precision LCZ meter with Kelvin Leads with 0 volts biasand 1 volt signal. The operating frequency was 100 kHz. The capacitanceand dissipation factor were measured using a Keithley 3330 Precision LCZmeter with Kelvin Leads with 2 volts bias and 1 volt signal. Theoperating frequency was 120 Hz and the temperature was 23° C.+2° C.

Leakage Current:

Leakage current (“DCL”) was measured using a MC 190 Leakage test setmade by Mantracourt Electronics LTD, UK. The MC 190 test measuresleakage current at a temperature of 25° C. and at a certain ratedvoltage after 10 seconds.

EXAMPLE 1

The ability to form a tantalum capacitor having a conductive polymerlayer formed from a polymerization solution was demonstrated. Inparticular, 50,000 μFV/g tantalum powder was pressed into pellets andsintered to form a porous electrode body. The pellets were anodized in aphosphoric acid electrolyte in water and subsequently shell formed inwater/ethylene glycol electrolyte to form the dielectric layer. Aprotective adhesive layer was applied to the porous electrode body froma solution of one part by weight polyvinylalcohol (Sigma-Aldrich Co.)and one part by weight methyl methacrylate (Sigma-Aldrich Co.) inninety-eight parts of water. The solution was formed by gently heatingto 70° C. The anode pellets were immersed in this solution and dried for15 minutes at a temperature of 100° C.

A polymerization solution was prepared to form the conductive polymercoating. The polymerization solution prepared with twelve parts byweight ethanol, 0.3 parts per weight methyl methacrylate(Sigma-AldrichCo.), 0.1 part by weight methyl pyrrolidone(Sigma-Aldrich Co.), 1 partby weight 3,4-ethylenedioxythiophene (sold under the name Baytron® M byBayer Corp.), and 10 parts by weight iron III tosylate in butanol (soldunder the name Baytron® CB40 by Bayer Corp.). The solution was used tocoat the anode pellets having the dielectric layer and the protectiveadhesive layer. The anode pellets were immersed in the polymerizationsolution cooled to 5° C. and kept under dry air. The monomers of thepolymerization solution were polymerized for one hour at ambienttemperature and 60% relative humidity. The anode pellets were immersedin the polymerization solution and polymerized a total of six times toform the conductive polymer layer.

Another polymerization solution was prepared with the same ingredientsas above in the first step, except it was diluted six times by weightethanol. The anode pellets having the conductive polymer layer wereimmersed in this solution while at 5° C. and kept under dry air andre-anodized. After re-anodization, the resulting pellets werepolymerized at ambient temperature and 60% relative humidity.

The pellets were then coated with a graphite coating and a silvercoating.

Finally, the ability to form a barrier layer on the resulting capacitorwas demonstrated. A solution comprising two parts by weight polyurethanediol (Sigma-Aldrich Co.) and two parts polyethyleneimine (Sigma-AldrichCo.) in ninety six parts of ethanol was produced. The pellets with suchcovered cathode were immersed in this solution and then dried at 25° C.for 30 min.

The finished parts were completed by conventional assembly technologyand measured.

EXAMPLE 2

50,000 μFV/g tantalum powder was pressed into pellets and sintered toform a porous electrode body. The pellets were anodized in a phosphoricacid electrolyte in water and subsequently shell formed inwater/ethylene glycol electrolyte. The porous electrode body was appliedwith a solution one part by weight 3,4-ethylenedioxythiophene (Baytron®M, H.C. Starck GmbH), twenty parts by weight iron(III) tosylate inbutanol (Baytron® CB40, H.C. Starck GmbH) and twelve parts by weightethanol. The solution was used to impregnate of anode pellets withpre-coated dielectrics. The anode pellets were immersed in thissolution, cooled at 5° C. and kept under dry air, and then polymerizedfor one hour at ambient temperature and 60% relative humidity. The anodepellets were immersed in the polymerization solution and polymerized atotal of six times to form the conductive polymer layer.

Another polymerization solution was prepared with the same ingredientsas above in the first step, except it was diluted six times by weightethanol. The anode pellets with polymeric layer were immersed in thissolution, cooled at 5° C. and kept under dry air, and re-anodized. Afterre-anodization these pellets were polymerized at ambient temperature and60% relative humidity. The pellets were then coated with a graphite andsilver coating. The finished parts were completed by conventionalassembly technology and measured.

The parameters of the samples made are shown in Table 1:

TABLE 1 Leakage Cap DF ESR Current Capacitor (μF) (%) (mμ) (μA) Example1 9.9 2.0 95 1.2 Example 2 9.8 1.9 122 2.0

The stability of the resulting capacitors was also tested after exposureto different environments. Specifically, samples from each Example 1 andthe Comparative Example were exposed to an environment of 85% relativehumidity at 85° C. for 1000 (as shown in Table 2), and changes in thecapacitor's performance were measured.

TABLE 2 % Change in Temp R.H. Time Leakage (° C.) (%) (hours) Cap DF ESRCurrent Example 1 85 85 1000 10 −10 20 0 Example 2 85 85 1000 −100 1000015000 0

The capacitors of Example 1, which have a barrier layer, were much morestable after aging in such environments, and showed much less change inproperties.

These and other modifications and variations of the present inventionmay be practiced by those of ordinary skill in the art, withoutdeparting from the spirit and scope of the present invention. Inaddition, it should be understood that aspects of the variousembodiments may be interchanged both in whole or in part. Furthermore,those of ordinary skill in the art will appreciate that the foregoingdescription is by way of example only, and is not intended to limit theinvention so further described in such appended claims.

1. A method for forming a solid electrolytic capacitor, the methodcomprising: forming an anode; anodizing a surface of the anode to form adielectric layer; and forming a conductive polymer coating over theanodized anode by polymerizing a monomer in the presence of an oxidativepolymerization catalyst, wherein less than a stoichiometric amount ofthe oxidative polymerization catalyst is present in the polymerizationsolution per mole of monomer.
 2. A method as in claim 1, wherein theconductive polymer coating is formed from a polymerization solutioncomprising the monomer, the oxidative polymerization catalyst, and apolar solvent.
 3. A method as in claim 2, wherein the polymerizationsolution further comprises a dopant.
 4. A method as in claim 3, whereinthe oxidative polymerization catalyst and the dopant are supplied fromthe same compound.
 5. A method as in claim 3, wherein the oxidativepolymerization catalyst and the dopant are supplied from an organic acidligand combined with iron III.
 6. A method as in claim 5, wherein theoxidative polymerization catalyst and the dopant are supplied from ironIII tosylate.
 7. A method as in claim 2, wherein the polar solventcomprises an aprotic polar solvent.
 8. A method as in claim 7, whereinthe aprotic solvent comprises N-methylpyrrolidone, dimethyl sulfoxide,dimethylformamide, hexamethylphosphorotriamide, dimethyl acetamide,methyl ethyl ketone, or mixtures thereof.
 9. A method as in claim 7,wherein the polar solvent comprises N-methylpyrrolidone.
 10. A method asin claim 2, wherein the polar solvent is combined with a co-solvent toform a solvent system.
 11. A method as in claim 10, wherein the weightratio of the co-solvent to the polar solvent is about 50:1 or more. 12.A method as in claim 1, wherein the conductive polymer comprisespolypyrroles, polythiophenes, polyanilines, polyacetylenes,poly-p-phenylenes, or mixtures or derivatives thereof.
 13. A method asin claim 1, wherein the conductive polymer coating comprisespoly(3,4-ethylenedioxy thiophene).
 14. A method as in claim 1, whereinthe oxidative polymerization catalyst comprises a transitional metalsalt derivatized with organic ligands.
 15. A method as in claim 1,wherein from less than about 15 moles of the oxidative polymerizationcatalyst are present per mole of monomer.
 16. A method as in claim 1,wherein from about 5 moles to about 12 moles of the oxidativepolymerization catalyst are present in the polymerization solution permole of monomer.
 17. A method as in claim 1, wherein about one half toabout three fourths of a stoichiometric amount of the oxidativepolymerization catalyst is present in the polymerization solution permole of monomer.
 18. A method as in claim 1, further comprising forminga protective adhesive layer between the anodized anode and theconductive polymer coating.
 19. A method as in claim 1, furthercomprising forming a barrier layer over the conductive polymer coating.20. A method as in claim 1, wherein the anode contains a valve metal.21. A method as in claim 20, wherein the anode contains tantalum or aniobium oxide.
 22. An solid electrolytic capacitor comprising an anodethat contains a valve metal; a dielectric layer formed over the anode; apoly(3,4-ethylenedioxy thiophene) coating formed over the anode, whereinthe poly(3,4-ethylenedioxy thiophene) coating is polymerized in thepresence of less than a stoichiometric amount of an oxidativepolymerization catalyst.
 23. A method as in claim 22, wherein the anodecontains tantalum or a niobium oxide.